Stays mostly, that is ladies' underwear (8)
I believe the answer is:
lingerie
'ladies' underwear' is the definition.
(I've seen this before)
'stays mostly that' is the wordplay.
'stays' becomes 'lingers' (lingering is a kind of staying).
'mostly' means to remove the last letter.
'that' becomes 'ie' (I am not sure about this - if you are sure you should believe this answer much more).
'lingers' with its last letter taken away is 'linger'.
'linger'+'ie'='LINGERIE'
'is' is the link.
Can you help me to learn more?
(Other definitions for lingerie that I've seen before include "pants! Could be" , "flimsy stuff" , "Women's underwear and nightclothes" , "Female underwear" , "CLOTHING" .)